A. Field of the Invention
The present invention relates to a lubricant for magnetic recording media, and to a magnetic recording medium for use in magnetic recording devices, in particular a magnetic recording medium for use in external memory devices of computers.
B. Description of the Related Art
A lubricant has been developed which is used in magnetic recording media, especially in magnetic discs for reducing the frictional force to occur between the protective layer thereof and a head and for improving the durability and the reliability thereof. For example, a perfluoropolyether-type lubricant having a polar terminal group such as a hydroxyl group or a cyclic triphosphazene terminal group in the molecule has been applied onto the diamond-like carbon (DLC) protective layer to improve the lubricating characteristics of the surface layer of a magnetic recording medium.
The surface of a diamond-like carbon (DLC) protective layer has a large number of functional groups such as a carboxyl group, a hydroxyl group and an amino group existing therein, and the functional groups positively adsorb and bond to the terminal groups of the above-mentioned lubricant. However, the functional groups have a characteristic of positively adsorbing pollutants such as moisture and acidic gas, as well as the lubricant terminal groups. Accordingly, in cases where the surface of the protective group has free functional groups which are not bonded to the lubricant terminal groups, the adsorption of pollutants may increase.
In order to prevent pollutants such as moisture and acidic gas from being adsorbed by the surface of a disc medium, it is necessary to increase as much as possible the bonding degree (bonding ratio) between the functional groups existing in the carbon surface and the lubricant. See, for example, JP-A-Hei-5-247200 and JP-A-2004-253110 (US family, US 2004/185262A1).
In general, applying a lubricant onto a carbon surface followed by heat treatment may increase the bonding degree between the functional groups on the carbon surface and the lubricant terminal groups. A higher heating temperature in the treatment promotes an increase in the bonding degree between them. However, it is limitative to increase the bonding degree between the functional groups existing in the carbon surface and the lubricant terminal groups. In particular, in the conventional technique of using a perfluoropolyether-type lubricant having a polar terminal group such as a hydroxyl group or a cyclic triphosphazene terminal group in the molecule, a phenomenon of “heating loss” caused by lubricant vaporization from a disc surface is remarkable when the heating temperature is over 100° C.; and this phenomenon becomes more remarkable when the heating temperature is higher. Accordingly, in the high-temperature treatment, the amount of the lubricant existing on a disc surface may be greatly reduced as compared with that before the heat treatment. Therefore, the increase in the bonding degree by heat treatment is limited.
In general, the bonding degree between the functional groups existing in a carbon surface and the terminal groups of a lubricant is represented by the ratio of the thickness of the lubricant layer after washing with a fluorine-containing solvent to the thickness of the lubricant layer before washing with the fluorine-containing solvent, and the percentage is referred to as “bonding ratio”:
      Bonding    ⁢                  ⁢          Ratio      ⁢                          [      %      ]        =                                                        lubricant              ⁢                                                          ⁢              layer              ⁢                                                          ⁢              thickness                                                                          after              ⁢                                                          ⁢              washing                                                                                      lubricant              ⁢                                                          ⁢              layer              ⁢                                                          ⁢              thickness                                                                          before              ⁢                                                          ⁢              washing                                            ×    100  
In this definition, the lubricant layer thickness before washing is referred to as “total lubricant layer thickness,” the lubricant layer thickness after washing is as “bonding lubricant layer thickness,” and the difference between the total lubricant layer thickness and the bonding lubricant layer thickness is the “free lubricant layer thickness.” The “bonding lubricant layer thickness” represents the thickness (amount) of the lubricant actually bonding to a carbon surface, and to inhibit the pollutant adsorption, this amount must be increased.
The “bonding ratio” standard is described. In cases where Vertrel XF (by Mitsui DuPont Fluorochemical) is used as a fluorine-containing solvent and when a perfluoropolyether-type lubricant having a polar terminal group such as a hydroxyl group or a cyclic triphosphazene terminal group in the molecule (e.g., Fomblin Z-Tetraol, by Solvay Solexis) of the prior art is used, the uppermost limit of the bonding ratio may be about 70% for the reasons mentioned above (JP-A-Hei-10-143838). In this case, when the total lubricant layer thickness is 1.0, 1.2 or 1.4 nm, which is, at present, an ordinary lubricant layer thickness in ordinary hard discs, then the bonding lubricant layer thickness is 0.7, 0.84 or 0.98 nm, respectively.
On the other hand, with the recent tendency toward high-density magnetic discs, the requirements of lubricant characteristics have become more severe. To satisfy the requirements, in future, the uppermost limit of the bonding lubricant layer thickness must be increased even more.
In addition, regarding the recent application mode of hard disc drives, not only the conventional mainstream uses thereof for indoor personal computers, but also their outdoor applications for mobile devices, car navigation systems and others, are increasing. In particular, often problematic is a phenomenon of difficult floating of magnetic head sliders in high-temperature high-humidity environments, and this may be the result of aggregation of moisture existing in high-humidity air. Accordingly, it is a significant problem how to reduce the amount of moisture that adheres to and aggregates on the surface of a magnetic disc, in other words, how to hydrophobicate the disc surface.
Hydrophobication requires an increase in the bonding lubricant layer thickness and, in cases where the uppermost limit of the bonding ratio is defined, a simple increase in the total lubricant layer thickness may be enough for the increase in the bonding lubricant layer thickness. However, a simple increase in the layer thickness may often bring about a phenomenon of lubricant pickup by the floating slider, therefore causing a problem of slider floating instability.
Accordingly, a method of increasing the bonding ratio between the lubricant and the carbon surface as compared with that in a conventional method is required instead of changing the total lubricant layer thickness therein.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.